Improve documentation for the `position` field of the `QSelf` type, which is difficult to understand.

[Luro02]

I struggled for quite a while to understand how to deal with the position field of the syn::QSelf type. I understand that as an author it is difficult to see parts in a project that are not understandable, so I made this issue.

I had a few questions that I think the documentation should answer: The docs mention that the as trait and associated item are stored separately, but there is no mention of where that is.

It also mentions that the position for AssociatedItem in <Vec<T> as a::b::Trait>::AssociatedItem is 3, but not how this number changes depending on the type. For example, what would be the position of the a::b::Trait be? How could one obtain this? An example code would be immensely helpful. I am not good with writing examples, but here is something as an idea:

use quote::ToTokens;
use syn::punctuated::Punctuated;

let ty: syn::TypePath = syn::parse_quote!(<Vec<T> as a::b::Trait>::AssociatedItem);

// the ty.path looks like this a::b::Trait::AssociatedItem
// therefore when a qualified self is involved, it can not
// be treated as a normal path.

// this checks if the ty is cast to a trait and if so, prints
// that trait
if let Some(syn::QSelf {
    ty: sub_ty,
    as_token,
    position,
    ..
}) = &ty.qself
{
    if as_token.is_some() {
        // the type has been cast to another trait
        // the position is the index where the associated item
        // starts in the ty.path
        //
        // everything up to position (0..position) is the trait
        // that it is cast to
        let mut trait_path = syn::Path {
            leading_colon: None,
            segments: Punctuated::new(),
        };

        for i in 0..*position {
            trait_path.segments.push(ty.path.segments[i].clone());
        }

        // a :: b :: Trait
        println!("the ty is cast to {}", trait_path.to_token_stream());
    } else {
        println!("ty has not been casted");
    }
}